1. Field
An embodiment of the present invention relates to an organic light emitting display.
2. Description of Related Art
Recently, various flat panel displays (FPDs) capable of reducing weight and volume, which are disadvantages of cathode ray tubes (CRTs), have been developed. The FPDs include liquid crystal displays (LCDs), field emission displays (FEDs), plasma display panels (PDPs), and organic light emitting displays.
Among the FPDs, the organic light emitting display displays an image using organic light emitting diodes (OLEDs) that generate light by the recombination of electrons and holes.
The organic light emitting displays are being widely applied in personal digital assistants (PDAs), MP3 players, and mobile telephones due to advantages such as excellent color reproducibility and reduced thickness.
FIG. 1 is a circuit diagram illustrating a pixel of an organic light emitting display. Referring to FIG. 1, the pixel is coupled to a data line Dm, a scan line Sn, and a pixel power source line coupled to a pixel power source ELVDD and includes a first transistor M1, a second transistor M2, a capacitor Cst, and an organic light emitting diode OLED.
In the first transistor M1, a source is coupled to the pixel power source line ELVDD, a drain is coupled to the OLED, and a gate is coupled to a first node N1. In the second transistor M2, a source is coupled to the data line Dm, a drain is coupled to the first node N1, and a gate is coupled to the scan line Sn. The capacitor Cst is coupled between the first node N1 and the pixel power source ELVDD to maintain a voltage between the first node N1 and the pixel power source ELVDD for an amount of time (e.g., a predetermined time). The OLED includes an anode electrode, a cathode electrode, and a light emitting layer. In the OLED, the anode electrode is coupled to the drain of the first transistor M1 and the cathode electrode is coupled to a low potential power source ELVSS, so that when current flows from the anode electrode to the cathode electrode, the light emitting layer emits light, and brightness is controlled corresponding to the amount of current.
In the pixel having the above structure, current corresponding to EQUATION 1 flows to the OLED.
                              I          d                =                              β            2                    ⁢                                    (                              Vgs                -                Vth                            )                                      2              =                                ⁢                      β            2                    ⁢                                    (                              ELVdd                -                Vdata                -                Vth                            )                        2                                              EQUATION        ⁢                                  ⁢        1            wherein, Id, Vgs, Vth, ELVdd, Vdata, and β represent current that flows to the OLED, a voltage between the gate and source of the first transistor, a threshold voltage of the first transistor, a voltage of the pixel power source, a voltage of the data signal, and a constant, respectively.
Since the current that flows to the OLED is as represented by EQUATION 1, when the voltage of the pixel power source ELVDD changes, the amount of current that flows also changes.
Therefore, since a magnitude of internal resistance of the pixel power source line to which the pixel power source ELVDD is coupled varies with a distance of the pixel from the pixel power source ELVDD, a difference in brightness between pixels may be generated.